1. Field of the Invention
The present invention relates to a document feeding apparatus for feeding documents stacked on a document stacker one by one onto a document glass plate and returning the documents onto the document stacker after an exposing processing so as to feed repeatedly the documents to the document glass plate.
2. Description of the Prior Art
There are two kinds of document feeding apparatuses, one is an ADF wherein documents stacked on a document stacker are separated and fed one by one onto a document glass plate, and discharged to an outside discharge tray after an exposing process for the documents on said document glass plate. The other is an RDF wherein documents stacked on a document stacker are separated and fed one by one onto a document glass plate, and returned onto the document stacker after an exposing processing for the documents on the document glass plate so that the documents can be fed repeatedly to the document glass plate.
In such document feeding apparatus the separating ability of the document feeding means is very important. A bottom feed system has been adopted as a system having a good separating ability. In such system, the documents are separated one by one by using suction or by using friction. The latter has advantages over the former in that in that it is low in the manufacturing cost, small in size and simple in adjustment and maintenance thereof.
FIG. 5 shows a conventional document feeding apparatus equipped with a document feeding device of frictional separating system as mentioned above. In FIG. 5, reference numeral 170 denotes a document stacker, 171 denotes a first document feeding means provided adjacent to said document stacker 170 for separating by friction documents stacked on said document stacker 170 one by one, and 174 denotes a second document feeding means provided in a path 173 communicating said first document feeding means 171 with a processing unit 172.
Said first document feeding means 171 consists of a feeding belt 171a driven by a drive source (not shown) through an electromagnetic clutch MC1, and a separating roller 171b which is in sliding contact with an upper surface of said feeding belt 171a in a non-rotary state, so that a sheet of documents at the lower most layer which is in direct contact with the feeding belt 171a can be separated by friction and fed when the feeding belt 171a is rotated in a feeding direction under such a state that the leading ends of the documents located on the feeding belt 171a are prevented from being moved by the separating roller 171b.
Said feeding belt 171a is stopped when the leading end of the forwarded document has passed through said second document feeding means 174 and reaches a synchronous sensor 175, but the trailing end of the document has not yet passed through a press contact point (a) formed between said separating roller 171b and said feeding belt 171a, so that the next sheet of documents is prevented from flying. The feeding of the documents to the processing unit 172 is continued by said second document feeding means 174. At the processing unit 172, an exposure is initiated after a predetermined time has lapsed from a time at which the leading end of said sheet of documents has passed through said synchronous sensor 175.
In the above apparatus, however, the frictional separation of the documents is carried out under the state that the leading ends of the documents are prevented from being forwarded by said separating roller 171b, so that the forwarding power is not so large and accordingly a time required for reaching the leading end of the document to said sensor 175 may be varied due to the stiffness of the document or the amount of the documents stacked on the document stacker 170. Accordingly, the distance between the documents becomes uneven so that jamming occurs and the duration of time between the detection of the leading end of the document by the sensor 175 and the initiation of the exposure are fluctuates.